Drafting apparatus



Dec. 27, 1966 w. FICKEN 3,293,756

DRAFTING APPARATUS Filed Deo. 1l, 1964 4 Sheets-Sheet vl Dec. Z7, 1966w. FICKEN 3,293,756

DRAFTING APPARATUS l Filed Dec. ll, 1964 4 Sheets-Sheet 2 INVENTQR.

Dec. 27, 1966 w. FICKEN DRAFTING APPARATUS 4 Sheets-Sheet 5 Filed Dec.l1, 1964 Dec. 27, 1966 w. PlcKl-:N

DRAFTING APPARATUS 4 Sheets-Sheet 4 Filed DeC- ll, 1954 IN VENTOR.

mlm@ .l P w mt' a MIZ United States Patent O 3,293,756 DRAFTINGAPPARATUS William Ficken, M9232 Peoria Ave., Sun City, Ariz. S5251 FiledDec. 1l, i964, Ser. No. 417,69) 5 Claims. (Cl. :i3-gil) The presentinvention relates to drafting apparatus and, more particularly, to anovel drafting machine and cornbination drafting board and draftingmachine.

Manyl types of drafting machines are presently availlable for attachmentto drafting boards to provide a movable straightedge above the workingsurface of the board for use as an adjustable guide for drawing straightlines or as an adjustable reference base upon which to supporttriangles, template-s or other drafting instruments. Although manycomplex and ingenious drafting machines have been developed to meet theneeds of the draftsman, and several types are widely used, all of thepreviously available drafting machines are subject to one or moreweil-known disadvantages. For example, in several popular types ofdrafting machines the straightedge is mounted on a bulky vertical armextending downward over the board from the top edge. Such vertical arms,which vary widely in design, for example, include boardlike membersseveral inches wide and rigid bar members with associated tensioningcables. Arms of these types are generally mounted to be movable fromside to side over the surface of the drafting board parallel to theupright sides thereof. Another type of arm in common use consists of abar and cable structure having two joints separated by an intermediateelbow-like articulation. Arms of this type are generally mounted byrotatably affixing the upper end to the top of the board so that thelower end of the arm, to which the straightedge is attached, is adaptedto be moved upward and downward and from side to side over the surfaceof the board. The se-veral foregoing types of vertical arms and otherssimilar in structure and purpose, have the disadvantage that theyoverlie the board and, due to their bulk, seriously interfere with thedraftsmans access to the drafting surface.

Other less complex drafting machines are also available in Which astraightedge disposed parallel to the horizontal edges of the board isattached at each end to an endless traveling cable. The traveling cable-at each end of the board is carried by a pair of wheels one of which isrotatably mounted at the top and one at t.e bottom of the adjacentvertical side of the board. The straightedge, while remaining parallelto the horizontal edges of the board is movable upward and downward overthe drafting surface in a direction normal to the horizontal edges ofthe boa-rd. Drafting machines of this type, while having the advantagethat the obscuring arm is obviated, are subject to the disadvantage thatthe straightedges do not provide a reliable reference base. This is dueto the fact that the traveling cables permit the straightedge to swayundesirably from side to side on the board.

All of the available drafting machines described above also have theserious disadvatage that they require counterweights of various types topermit their use when the drafting board is tilted at an angle of morethan to from the horizontal. Inasmuch as it is generally necessary totilt the drafting board at an angle of about 15 to 85 in order toprovide comfortable access to the board, particularly if the draftsrnanis to Work in a sitting position, it is obvious that the use ofcumbersome and diflicultly adjustable counterweights is necessary withavailable drafting machines under virtually yall normal use conditions.

Therefore, it is an object of the present invention to provide draftingmachines and combination drafting Mice boards and machines havingimproved -access 4to the drafting surface.

It is another object of the invention to eliminate the bulky verticalarms of previously available drafting machines.

It is still another object of the invention to eliminate the travelingcable of previously available drafting machines.

It is another object of the invention to provide a d-rafting machinehaving a horizontally disposed straightedge which is not subject to sidesway i.e. movement in the direction of its long axis.

It is yet another object of the invention to provide a combinationdrafting board and machine having a horizontally disposed straightedgewhich is movable upward and downward over the drafting surface but whichis adapted to remain where placed, irrespective of the forces ofgravity, and to be locked in any desired position.

lt is another object of the invention to provide a combination draftingboard and machine wherein the board is in the form of a rectangle havinga long axis and a short axis, means for supporting the board in Workingposition with either the long axis or short axis thereof disposedvertically as desired, and straightedge means adapted to be disposed ina horizontal position over the board when it is arranged with either thelong or short axis vertical.

It is still another object of the invention to provide a combinationdrafting board and machine including means for adjustably supporting theboard at any desired working angle from about 15 to 85 from thehorizontal and wherein the drafting machine is operable with the boardpositioned at such angles without the use of counterweights.

It is yet another object of the invention to provide a combinationdrafting board and machine whe-rein the board is faced with a syntheticplastic material capable of receiving and holding a compass pointwithout slipping and which is also adapted to hold a sheet of draftingpaper or the like thereagainst by electrostatic attraction.

It is another object of the invention to provide a combination draftingboard and machine wherein the horizontal and vertical edges of thedrafting surface of the board are provided with graduated scales andwhere-in the straightedge is also provided with a graduated scale, thegraduations of which correspond to the graduations on the edges of thedrafting surface of the board parallel to said straightedge, whereby allmeasurement reference points move with the movable straightedge thusobviating repeated reference to the graduated scales at the edge of theboard.

The above and other objects of this invention will be understood from areading of the following description of an embodiment thereof as shownin the accompanying drawings in which:

FIG. l is a top plan View of a combination drafting board and machine ofthe invention showing a movable straightedge disposed horizontally andparallel to the long axis of the board;

FIG. 2 is an elevational vie-W of the board resting on a horizontalsurface and showing the long side of the board which is uppermost inFIG. l;

FiG. 3 is a cross sectional view through a. roller assembly forming partof the drafting machine taken on the line 3--3` of FIG. l;

FIG. 4 is a cross sectional view of the roller assembly taken on theline 4-4 of FIG. 3 and showin-g a portion of the track on which theroller assembly runs;

FlG. 5 is another cross sectional view of the roller assembly taken onthe line 5-5 of FIG. 4;

FIG. 6 is a cross sectional view of the roller assembly taken on theline 6 6 of FIG. 4;

feg, es

FlG. 7 is a bottom plan view of thedrafting board and machine showingthe arrangement of brackets for attachinent of meansl for' 'supprtingthe board in working posinon;

FIG; Slis a side elevational View showing the drafting board and machineof FIG. 1 mounted in two different positions on adjustable supportmeans;

IFIG. 9 is a view partly in section of a supporting bracket taken alongthe line 9 9 of FIG. 8 and showing an instrument tray mounted on saidbracket; and

FIG. l0 is across sectional view of means for attaching the board to thesupport struct-ure taken along the line 10516 of FIG. 8; l

Similar characters of reference are employed in the above describedviews, to indicate ycorresponding parts.

Referring to the drawings, the reference character 1? indicates thedrawing board of the combination drafting 'board and machine. Saiddrawing board is rectangular in shape', having two parallel lo-ng Isidesand, at right angles thereto, two parallel short sides. The top surfaceof the board 1@ has bonded thereto a thin layer of a suitablethermoplastic material which provides a smooth drafting surface 11. Thedrafting surface 11 is hard to the touch but is ladapted to accept thepoint of a compass or other drafting instrument without slippage and isalso charac-` terized by having electrostatic attraction for paper andother sheet materials used by idraftsrnen. For this reason a sheet ofpaper or the like may be spread on the surface 11 and smoothed out withthe hand or a straight edge so that the paper will adhere t'o thesurface 11 s-ufciently to permit the draftsman to work without the useof thumb tacks or tape to hold the work in place. However, when the workis finished, the paper is easily removed by simply lifting a corner ofthe sheet and peeling it off the surface 11. The edges of the draftingsurface 11 of the board 10 are provided with suitable graduated scalesas shown in FIG. 1.

A straightedge member 12 .composed of a transparent plastic material ismounted for movement upward and downward in sliding contact with thedrafting surface 11 with its upper and lower straight edges parallel tothe long sides of the board. Another shorter straightedge member 12 (notshown) is also provided for alternative use mounted parallel to theshort sides of the board 10 when it is desired to use the board with itslong axis upright rather than horizontal as shown in FG. 1.

The straightedge member 12 is mounted on the board 10 'by attachment ofeach end to a roller assembly by means of a bolt 13, the threaded end ofwhich bolt extends through a bore hole located on the yaxis and near theend of the straightedge member 12. A washer 14 is placed on the end ofthe bolt 13 protruding through the hole in straightedge 12 and theassembly is secured by means of a knurled, internally threaded nut 15screwed on to the end of bolt 13. Each end of the straightedge 12 isthus rmly secured in a bedding channel provided therefor in each rollerassembly as |best shown in FIGS. 5 yand 6.l

The two roller assemblies, which are identical except that that each isthe mirror image of the other, are mounted at opposite sides of theboard on tracks 16. The tracks 16 are metal members having an angularC-shaped cr-oss section consisting of two legs 17 and 18 joined at a 90angle to ian intermediate base 19. The plane of the base 19 lof eachtrack 16 -is disposed parallel to the plane of the side, and normal tothe surface 11 of board 10. The planes of the legs 17 and 18 areparallel to the plane of the drafting surface 11 of the board. Each ofthe C-shaped tracks 16 is fixed to the board with its leg 17 spaced fromthe back of the board. The other leg 18 is nested lin the edge of (theupper surface of the board 10, which is suitably cut away for thispurpose las shown in FIG. 3 so that the upper surface of the leg 18 isilush with the upper surface of the board beneath the plastic draftingsurface 11. The exterior surface of base 19 and the inner edge 20 of leg17 of each track 16 function as bearing surfaces on which the rollerassembly moves.

track 16 is similarly mounted on each of the four sides of the board;the ends of adjacent tracks meeting at the corners of the board beingmitered at a 45D angle as shown in FlG. 7. The four angular C-snapedtracks 16 thus form a frame for the board and since they are of metalserve to protect the sidesof the board from dam-age.

The roller assemblies include a roller housing consist ing of a topplate 21, a bott-om plate 22 and a pair of channelshaped end plates 23which form a box-like structure open on two sides. A pair of guideplates 24 are fixed to the top plate 21 in spaced apart relation to formthe bedding channel -in which the end of a straightedge member 12 isaccommodated. A relatively large diameter main roller 2S of substantialthickness is mounted for rotation within the roller housing on bolt 13which passes through an axial bore hole in the roller. The bolt 13,which functions as a stationary axle for the main roller 25, has anenlarged head with a at lower surface. The bolt 13 passes through a borehole in 'bottom plate 22 of the roller housing and is fixed thereto bymeans of nut '.26 threaded thereon to pull the hat under surface of theenlarged hea-d of the bolt tightly against the plate 22. The other endof the bolt 13, vwhich extends thro-ugh bore holes in top plate 21 andstraightedge member 12, and protrudes therefrom, is secured 4asdescribed above by washer 14 and knurled nut 15 threaded thereon;

Each roller assembly is provided with a slide plate 27 one end of whichis iixed to bottom plate 22 of the main roller housing by bolt 13 andnut 26. The other end of the slide plate 27 extends beyond the plate 22in a direction normal -to the bolt and inboard of the roller 25. Theprotruding end Iof the slide plate 27 is adapted to slide into a slotina slide block 281; the slot and slide plate being correspondinglydimensioned to provide a close sliding lit therebetween. The slide block2S is made up of a bottom plate 29 and a top plate 3G suitably fastenedtogether. The bottom plate 29 has a cut-out channel in its upper surfacewhich when covered by the top plate 30 provides the aforementioned slotwhich extends through the slide b-lock. A pair of auxiliary rollers 31are rotatably mounted above the top plate 30 of the slide block inspaced apart relation by the use of bolts 32. Each of the bolts 32extends through an axial hole in one of the auxiliary rollers 31 andthrough vsuitable holes in the plates 29 and 30 of the slide block; theprotruding end of the ibolt in each case being secured by a nut threadedthereon.

The position of the slide plate 27 within the slot in the slide block isadjust-able and may be locked at a desired position. This isaccomplished by the use of a pair of knurled head set screws 33 having aconical apex iat the end opposite the knurled head Ias best seen inFIGS. 3 and 6. Two internally threaded bore holes, located in thechannel of the slide block with their centers on a line normal to theaxis of the main roller 25 extend through the bottom plate 29 to receivethe `set screws 33. Corresponding bore holes extending through the slideplate 27 have conical countersunk orices on the under side of the plateto receive the correspondingly shaped conical ends of the set screws 33.

Thus the roller assembly consists of two separable units, one consistingof the main roller 25 with its housing and protruding silde plate 27yand the other consisting of the slide block 28 carrying the auxiliaryrollers 31. The bore holes in the bottom plate 29 of the slide blocksand in the slide plate 27 are located so as to be in axial alignmentwhen the Set screws 33 are screwed into said bore holes to the fullestpossible extent. In this condition the auxiliary rollers 31 are disposedat a predetermined distance from the main roller 25. This distancebetween the main roller 25 and the auxiliary rollers 31 is such thatwhen the roller assembly is mounted on a track 15 and the set screws 33are turned up linger pressure tight, the main roller 25 is held firmlyagainst the exterior flat surface of base 19 of the track and theauxiliary rollers 31 are also held firmly against the narrow inboardedge 20 of leg 17 of the track. When the set screws 33 are thustightened in the bore holes in the bottom plate 29 of the slide blockand the countersunk holes in the lower surface of the slide plate 27,the clamping pressure of the rollers 25 and 31 on the track 16 is suchthat the roller assemblies on either side of the board 1t) and theconnecting straightedge member can be rolled upward and downward on theboard only by strong manual force.

The amount of force required to roll the drafting machine upward anddownward on the board can be reduced by lessening the clamping pressureof the rollers against the track. This is accomplished by simplyunscrewing the knurled set screws 33 slightly thus partially withdrawingthe conical points of the set screws from the conical countersunkorifices of the bore holes in the slide plate 27. When this is done thepoint of the conical end of the set screw 33 is no longer closelyconfined by the complementary point of the countersunk recess in theunderside of the slide plat-e 27 thus permitting slight relativemovement between the slide block and the slide plate. This slightdisplacement of the slide block increases the distance between the mainroller 25 and the auxiliary rollers 31 thus reducing the clampingpressure on the track and permitting movement of the drafting machineover the board with less effort. lt will be seen, therefore, that anydesired degree of ease of movement of the drafting machine can beachieved by simple adjustment of the set screws 33.

A brake assembly is provided for locking the drafting machine in adesired position. This asembly consists of a brake member 34 mountedadjacent to the main roller 25 inside the roller housing. The brakemember 34 is fixed to a knurled headed set screw 35 the threaded shaftof which extends through an internally threaded bore hole in the endplate 23 of the roller housing. When it is desired to lock the draftingmachine in a selected position the set screw 35 is manipulated to forcethe brake member 34 against the roller 25 thus effectively preventingtravel of the roller assembly on the track 16. When it is desired toshift the position of the drafting machine once again the brake isreleased 4by unscrewing the set screw 3S to withdraw the brake member 34from contact with the roller 25.

The combination drafting board and machine is provided with suitablemeans for supporting the board in substantially any desired workingposition. Four short rubber-tipped legs 36 are fixed to the back surfaceof the board as best seen in FIGS. 2 and 7 for use in supporting theboard on a fiat surface such as a table top. The legs 36 are positionedinboard from the sides of the board t0 prevent interference with theroller assemblies and are dimensioned to provide sufficient clearancebetween the roller assemblies and any fiat surface on which the board isplaced to permit use of the drafting machin-e.

The combination drafting board and machine is also provided withdetachable means for adjustably supporting the board at any desiredangle from about to 85 above a horizontal surface such as a table top.This support means consists of a pair of adjustable structures one ofwhich is detachably mounted inboard of and parallel to each of the shortsides of the board, for example, when the board is to be supported withits long axis horizontal. Each of the detachable support structuresconsists of a base member 37 having a rubber cushioning member 38 fixedto the underside near each end. Each base member 37 has an upstandingbracket 39 fixed thereto near one end and an adjustably extensible arm4@ rotatably fixed to the other end. The extensible arm 4t? includes anelongated hollow tube-like member of rectangular cross section and anelongated extensible member slidably fitted in said tube. A threadedbolt is fixed to the lower end of the inner extensible member of the arm40 so as to protrude through a longitudinal slot in the upper side ofthe elongated tube-like member. The overall length of the extensible arm40 is adjustably set by lplacing a washer over the bolt on the innerslidable member and threading a knurled cap nut 41 onto the end of saidbolt. When cap nut 41 is turned down finger pressure tight the inner andouter slidable members of arm 40 are held against sliding contact, thuseffectively locking the extensible arm at the desired length.

The two adjustable support structures are attached to the back of theboard by first engaging a pin at the upper end of bracket 39 in akey-slotted mounting bracket 42 fixed to the back of the board near thelower corner at one side. The upper end of the extensible arm 40 is thenfitted between the upstanding sides of a mounting bracket 43 fixed tothe back of the board on the same side near the upper corner. A knurledheaded, threaded pin 44 is then screwed into an internally threaded holeextending through the bracket 43 and arm 40.

When an adjustable support has been mounted on each side of the board inthis manner the tilt or angle of the board is adjustable from about 15to 85 when the base members 37 are placed on a horizontal surface suchas a table top. The angle of the board is, of course, determinedprimarily by the length of the extensible arm 40. When arm 40 is fullytelescoped as shown in full lines in FIG. 8, the board is disposed at anangle of about 15. To increase the slope of the board to any desiredangle up to about tne knurled caps 41 are loosened, the board raised tothe desired angle by extension of the arms 4i), and the cap nuts 41tightened again to lock the board at the desired position. Such anelevated position is shown in bro-ken lines in FIG. 8.

An upstanding bracket 39 of the support structure is provided with arectangular cut-out 45 in each side of the bracket to receive the shaft45 supporting an instrument tray 47 which is provided to hold thedraftsmans tools, pencils, erasers, ink and the like.

As noted previously, the combination drafting board and machine of thisinvention is adapted to be used with either the long or short axis ofthe rectangular board in an upright position. Therefore, two sets ofmounting brackets are fixed to the back of the board for attachment ofthe adjustable support structure. Referring to FIG. 7, it may be seenthat the brackets 42 and 43 on each side of the board connected by thelines a-a are employed to mount the board with its long axis horizontal.The brackets 42 and 43 connected by the lines b-b, on the other hand,are employed to mount the board with its long axis vertical.

It will -be apparent from the detailed description above of anembodiment of the present invention that the new drafting machines andcombination drafting boards and machines obviate many of thedisadvantages of the prior art devices. For example, inasmuch as theroller assemblie-s are mounted completely outboard of the draftingsurface of the drawing board, the draftsmans access to that surface ismaximized; the onl element present on the drafting surface beingstraightedge member which is a drafting tool. The new drafting machinesalso eliminate the need for the bulky vertical arms of previouslyavailable machines. Moreover, inasmuch as the side mounted travelingcables of the prior art have been eliminated and the straightedge memberis firmly mounted -between two unyiel-ding roller assemblies, thestraightedge is not subject to side sway and thus provides a dependablescale of reference points along its axis. Still further, provision foradjustment of the resistance to movement of the straightedge member bythe use of easily manipulated set screws permits setting of thisresistance so that the straightedge will remain where placed on theboard despite the force of gravity at any desired working angle of theboard. This leaves both of the draftsmans hands ree for the manipulationof instruments. The provision of the brake device also permits thestraightedge to be anchored at a desired position as a reference basefor any desired period of time and yet per-mits it to be readily shiftedto a new position `by simply releasing the brake.

Still another major advantage of the new drafting machine is that it canbe used on a board disposed at any desired working angle without the useof cumbersome and difficulty adjustable counterweights.

Another unique advantage of the drafting board and machine shown in theaccompanying drawings is that it provides the convenience of travelingreference points, i.e. inasmuch as the straightedge member is graduatedwith a scale identical to and in vertical registration with the scaleson the parallel edges of the drafting surface, it is in effect amoveable extension of these scales. Therefore, when it is necessary tomove the straightedge upward or downward over the board to Work at a newlocation, all of the reference points move with the sltraightedge andthe draftsman is not required to refer to the edge of the boardv -butmay@ take his reference directly from the straightedge member.

Not the least of the advantages of the new drafting apparatus is itssimplicity of manufacture and assembly which makes it substantially moreeconomical than comparable apparatus previously available.

Although the invention has been described in detail in connection with aspecific em-bodiment thereof shown in the drawings, this embodiment ismerely illustrative of others that will be apparent to those skilled inthe art and is not to be construed as limiting the scope of theinvention which is defined in the accompany claims.

I claim:

1i In a drafting machine, which when in use is -mounted on a boardhaving a planar drafting surface and at least two opposite and parallelstraight sides, comprising two roller means to roll along the oppositesides of the board as a track, one roller on each side, and an elongatedstraightedge means extending between and connected at each end to one ofsaid roller means, whereby sai-d straightedge means is mounted insliding contact With the drafting surface of such a Iboard for movementthereover in a direction normal to the long axis of the straightedgemeans, the improvement which comprises an auxiliary straight track meansmounted on the board spaced from and parallel to each of said oppositesides, and wherein :said roller means each include a main roller and atleast one auxiliary roller spaced therefrom, said roller means beingmoveably mounted on the -board by clamping engagement between said mainroller disposed in rolling contact with one of the opposite sides of theboard and an auxiliary roller disposed in rolling contact with saidauxiliary track.

2. A drafting machine'according to claim 1 having means for adjustingthe clamping force of said roller means.

3. A drafting machine according to claim 2, wherein the means foradjusting the clamping force of said roller means comprises a slideplate connected to and prof truding from said main roller, a slide blockconnected tol said slide roller and having a channel therein to receivesaid slide plate, a set screw having a conical apex, said set screwextending through and in threaded engagement with said slide block, andan orifice surrounded -by a conical countersunk surface in said slideplate, said set screw being positioned with its apex adjacent to `saidorifice in the slide plate, whereby on turning the set screw in onedirection its apex is advanced into said orifice and into contact withsaid countersunk conical surface and on turning the set screw in theopposite direction the apex thereof is withdrawn from said orifice, saidcontact between the apex of said set screw and said conical countersunksurface causing said slide plate to move in said channel with respect tosaid slide block, thus changing the distance ybetween said -main rollerand said auxiliary roller, whereby the clamping force of the rollermeans is adjusted.

fl. A drafting machine, adapted to be mounted on a board having a planardrafting surface and at least two opposite and parallel straight sides,which comprises in combination, a pair of roller means, one of saidroller means being adapted to be mounted to roll along each of theopposite and parallel sides of a Iboard as a main track, auxiliarystraight track means adapted to -be mounted spaced from and parallel tothe two opposite and parallel straight sides of a board, and anelongated straightedge means connected at each and thereof to one ofsaid roller means, said roller means each including a main roller and atleast one auxiliary roller, said roller means being moveably mountableby clamping engagement between said main roller disposed in rollingcontact with a slide of a board and an auxiliary roller in rollingcontact with said auxiliary track. with said auxiliary track.

5. A drafting machine according to claim 4 having means for adjustingthe clamping force of said roller means.

References Cited by the Examiner UNITED STATES PATENTS 611,697 10/1898Lundy 33-76 1,270,609 6/1918 Ezra. 1,440,993 l/ 1923 Hasbrouck et al.2,627,323 2/1953 Larson 33--76 X 2,805,479 9/1957 Droste 33-80 2,874,4742/1959 Eeele et al. 33--76 2,954,609 10/1960 Goertz 33-80 LEONARDFORMAN, Primary Examiner.

HARRY N. HAROIAN, Assistant Examiner.

1. IN A DRAFTING MACHINE, WHICH WHEN IN USE IS MOUNTED ON A BOARD HAVINGA PLANAR DRAFTING SURFACE AND AT LEAST TWO OPPOSITE AND PARALLELSTRAIGHT SIDES, COMPRISING TWO ROLLER MEANS TO ROLL ALONG THE OPPOSITESIDES OF THE BOARD AS A TRACK, ONE ROLLER ON EACH SIDE, AND AN ELONGATEDSTRAIGHTEDGE MEANS EXTENDING BETWEEN AND CONNECTED AT EACH END TO ONE OFSAID ROLLER MEANS, WHEREBY SAID STRAIGHTEDGE MEANS IS MOUNTED IN SLIDINGCONTACT WITH THE DRAFTING SURFACE OF SUCH A BOARD FOR MOVEMENT THEREOVERIN A DIRECTION NORMAL TO THE LONG AXIS OF THE STRAIGHTEDGE MEANS, THEIMPROVEMENT WHICH COMPRISES AN AUXILIARY STRAIGHT TRACK MEANS MOUNTED ONTHE BOARD SPACED FROM AND PARALLEL TO EACH OF SAID OPPOSITE SIDES, ANDWHEREIN SAID ROLLER MEANS EACH INCLUDE A MAIN ROLLER AND AT LEAST ONEAUXILIARY ROLLER SPACED THEREFROM, SAID ROLLER MEANS BEING MOVEABLYMOUNTED ON THE BOARD BY CLAMPING ENGAGEMENT BETWEEN SAID MAIN ROLLERDISPOSED IN ROLLING CONTACT WITH ONE OF THE OPPOSITE SIDES OF THE BOARDAND AN AUXILIARY ROLLER DISPOSED IN ROLLING CONTACT WITH SAID AUXILIARYTRACK.